I think this argument potentially confuses a technical with a political issue.
My view is that in both contexts modularity is good. In the licensing/political context, having modularity means that 2 pieces of software, or subsystems, or systems under 2 different free licenses or under free and proprietary licenses can communicate. Modularity makes competition between different implementations possible and evolution ensures survival of best working code (usually but not always free).
If a proprietary hardware manufacturer which doesn't want to disclose information to competitors through support of free software drivers can support Linux through a userspace driver this means those who have bought, or want to buy this hardware won't be totally discouraged by the lack of Linux support for it. If a second hardware manufacturer can achieve better performance with a GPL kernelspace driver this has the potential advantage of more Linux sales than the first manufacturers products. Unless there is competition for Linux related sales between them, neither manufacturer need bother.
Extending the argument to reductio ad absurdam, consider the case that the ideological purity of Linux requires that it doesn't communicate with any _system_ that uses non-free software, including Windows boxes using TCP/IP. In practice there are different levels of communication possible between software components, and genuine technical modularity and neutral and published interfacing standards such as TCP/IP not only leads to more reliable components and systems such as Linux and BSD, but this also creates niches leading to whole market ecosystems within which free software can flourish and grow.
So I think that having support for a generic Linux userspace device driver interface is an excellent idea, both for technical reasons in the sense of introducing a boundary protecting the overall system from buggy userspace code, and for the purpose of advancing the cause of free software as a political benefit through the incremental improvement this technical interface allows in the free software marketplace.
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